The demand for sodium chloride of electrolytic cell quality is increasing. Plants which consume large amounts of this commodity necessarily have relied on supply sources which are not so distant that high shipping costs must be borne. However, not all such supply operations can be expanded to meet increasing demands for cell quality salt. Thus, for example, there may not be room to build additional solar evaporation ponds in an area which has been developed for other purposes around an established evaporation facility. Or, as another example, environmental considerations may rule out disposal of increased amounts of bitterns, mining spoils, etc. in the vicinity of a given salt processing operation.
Consequently, it is becoming increasingly necessary to consider utilizing extant salt bodies which do not meet electrolytic cell requirements but which can be upgraded and are located in areas subject to less constraints. Such deposits are generally more remote from electrochemical plants and their utilization necessarily entails greater freighting costs. It is then essential to devise a minimally expensive method of upgrading the raw salt obtained from these lower purity deposits.
Exemplary of existing impure salt bodies of considerable volume are those which have been deposited in primary evaporation ponds, adjacent to the western edge of the Great Salt Lake Basin, Utah, during processing of local brines for potassium chloride recovery. These deposits have sulfate ion contents of from about 0.6% to 1.2% by weight, as compared to typical sulfate contents of 0.15 to 0.3% for salt produced by evaporation of sea water. In view of the distance of these deposits from the majority of potential consumers, a cheap method of sulfate removal is imperative to their utilization.
Such references to NaCl purification as have been found in the literature are concerned with processes, such as fractional crystallization or selective precipitation, which are carried out on salt solutions. The most commonly employed method of removing dissolved sulfate is by calcium chloride addition to precipitate it as calcium sulfate. Application of this method to millions of tons of solid salt would require corresponding amounts of both water and calcium chloride. Extensive new evaporation ponds or a considerable expenditure of energy for subsequent water removal would also be required. Consequently, this method, and other solution processes - which would have similar requirements, are not attractive. Thus, a need for an efficient and economical method of reducing the sulfate contents of high sulfate salt deposits is evident.